‘Mitochondrial Eve’ ~ To be or not to be?

Mitochondrial Eve is estimated to have lived around 200,000 years ago, most likely in East Africa, when Homo sapiens sapiens (anatomically modern humans) were developing as a population distinct from other human sub-species.

On the first day of 1987, a scientific “discovery” seized the attention of the popular press. The original scientific article that caused all the commotion — “Mitochondrial DNA and Human Evolution”— appeared in the January 1, 1987 issue of Nature, and was authored by Rebecca Cann, Mark Stoneking, and Allan C. Wilson (see Cann, et al., 1987). These three scientists announced that they had “proven” that all modern human beings can trace their ancestry back to a single woman who lived 200,000 years ago in Africa.

This one woman was nicknamed “Eve” (a.k.a., “mitochondrial Eve”)—much to the media’s delight. An article in the January 26, 1987 issue of Time magazine bore the headline, “Everyone’s Genealogical Mother: Biologists Speculate that ‘Eve’ Lived in Sub-Saharan Africa” (Lemonick, 1987).

A year later, that “speculation” became a major Newsweek production titled, “The Search for Adam and Eve” (Tierney, et al., 1988). The provocative front cover presented a snake, tree, and a nude African couple in a “Garden of Eden” type setting. The biblical-story imagery was reinforced by showing the woman offering an apple to the man.

A word of explanation is in order. For decades, evolutionists had been trying to determine the specific geographical origin of humans—whether we all came from one specific locale, or whether there were many small pockets of people placed around the globe. When they set out to determine the specific geographical origin of humans, a curious piece of data came to light. As they considered various human populations, Africans seemed to show much more genetic variation than non-Africans (i.e., Asians, Europeans, Native Americans, Pacific Islanders, et al.). According to molecular biologists, this increased variability is the result of African populations being older, thus, having had more time to accumulate mutations and diverge from one another. This assumption led some researchers to postulate that Africa was the ancient “cradle of civilization” from which all of humanity had emerged.

In the field of human genetics, Mitochondrial Eve refers to the matrilineal most recent common ancestor (MRCA) of modern humans.

In other words, she was the woman from whom all living humans today descend, on their mother’s side, and through the mothers of those mothers and so on, back until all lines converge on one person.

Because all mitochondrial DNA (mtDNA) is generally passed from mother to offspring without recombination, all mitochondrial DNA (mtDNA) in every living person is directly descended from hers by definition. Mitochondrial Eve is the female counterpart of Y-chromosomal Adam, the patrilineal most recent common ancestor, although they lived thousands of years apart.

Each ancestor (of people now living) in the line back to the matrilineal MRCA had female contemporaries such as sisters, female cousins, etc. and some of these female contemporaries may have descendants living now (with one or more males in their descendancy line). But none of the female contemporaries of the “Mitochondrial Eve” has descendants living now in an unbroken female line.

The genetic material (DNA) in a cell’s nucleus controls the functions of the cell, bringing in nutrients from the body and making hormones, proteins, and other chemicals. Outside the nucleus is an area known as the cytoplasmic matrix (generally referred to simply as the cytoplasm), which contains, among other things, tiny bean-shaped organelles known as mitochondria. These often are described as the “energy factories” of the cell.

Although our mtDNA should be, in theory at least, the same as our mother’s mtDNA, small changes (or mutations) in the genetic code can, and do, arise. On rare occasions, mutations are serious enough to do harm. More frequently, however, the mutations have no effect on the proper functioning of either the DNA or the mitochondria. In such cases, the mutational changes will be preserved and carried on to succeeding generations.

Theoretically, if scientists could look farther and farther into the past, they would find that the number of women who contributed the modern varieties of mitochondrial DNA gets less and less until, finally, we arrive at one “original” mother. She, then, would be the only woman out of all the women living in her day to have a daughter in every generation till the present. Coming forward in time, we would see that the mtDNA varieties found within her female contemporaries were gradually eliminated as their daughters did not have children, had only sons, or had daughters who did not have daughters. This does not mean, of course, that we would look like this putative ancestral mother; rather, it means only that we would have gotten our mitochondrial DNA from her.

To find this woman, researchers compared the different varieties of mtDNA in the human family. Since mtDNA occurs in fairly small quantities, and since the researchers wanted as large a sample as possible from each person, they decided to use human placentas as their source of the mtDNA. So, Rebecca Cann and her colleagues selected 145 pregnant women and two cell lines representing the five major geographic regions: 20 Africans, 34 Asians, 46 Caucasians, 21 aboriginal Australians, and 26 aboriginal New Guineans (Cann, et al., 1987, 325:32). All placentas from the first three groups came from babies born in American hospitals. Only two of the 20 Africans were born in Africa.

Mitochondrial Eve lived later than Homo heidelbergensis and the emergence of Homo neanderthalensis, but earlier than the out of Africa migration. The dating for ‘Eve’ was a blow to the multiregional hypothesis, and a boost to the hypothesis that modern humans originated relatively recently in Africa and spread from there, replacing more “archaic” human populations such as Neanderthals. As a result, the latter hypothesis became dominant.

Female and mitochondrial ancestry

Without a DNA sample, it is not possible to reconstruct the complete genetic makeup (genome) of any individual who died very long ago. By analysing descendants’ DNA, however, parts of ancestral genomes are estimated by scientists. Mitochondrial DNA (mtDNA) and Y-chromosome DNA are commonly used to trace ancestry in this manner. mtDNA is generally passed un-mixed from mothers to children of both sexes, along the maternal line, or matrilineally. Matrilineal descent goes back to our mothers, to their mothers, until all female lineages converge.

Branches are identified by one or more unique markers which give a mitochondrial “DNA signature” or “haplotype” (e.g. the CRS is a haplotype). Each marker is a DNA base-pair that has resulted from an SNP mutation. Scientists sort mitochondrial DNA results into more or less related groups, with more or less recent common ancestors. This leads to the construction of a DNA family tree where the branches are in biological terms clades, and the common ancestors such as Mitochondrial Eve sit at branching points in this tree. Major branches are said to define a haplogroup (e.g. CRS belongs to haplogroup H), and large branches containing several haplogroups are called “macro-haplogroups”.

The mitochondrial clade which Mitochondrial Eve defines is the species Homo sapiens sapiens itself, or at least the current population or “chronospecies” as it exists today. In principle, earlier Eves can also be defined going beyond the species, for example one who is ancestral to both modern humanity and Neanderthals, or, further back, an “Eve” ancestral to all members of genus Homo and chimpanzees in genus Pan. According to current nomenclature, Mitochondrial Eve’s haplogroup was within mitochondrial haplogroup L because this macro-haplogroup contains all surviving human mitochondrial lineages today, and she must predate the emergence of L0.

The variation of mitochondrial DNA between different people can be used to estimate the time back to a common ancestor, such as Mitochondrial Eve. This works because, along any particular line of descent, mitochondrial DNA accumulates mutations at the rate of approximately one every 3,500 years. A certain number of these new variants will survive into modern times and be identifiable as distinct lineages. At the same time some branches, including even very old ones, come to an end, when the last family in a distinct branch has no daughters.

Mitochondrial Eve is the most recent common matrilineal ancestor for all modern humans. Whenever one of the two most ancient branch lines dies out, the MRCA will move to a more recent female ancestor, always the most recent mother to have more than one daughter with living maternal line descendants alive today. The number of mutations that can be found distinguishing modern people is determined by two criteria: firstly and most obviously, the time back to her, but secondly and less obviously by the varying rates at which new branches have come into existence and old branches have become extinct. By looking at the number of mutations which have been accumulated in different branches of this family tree, and looking at which geographical regions have the widest range of least related branches, the region where Eve lived can be proposed.

The date when Mitochondrial Eve lived is estimated by determining the MRCA of a sample of mtDNA lineages. In 1980, Brown first proposed that modern humans possessed a mitochondrial common ancestor that may have lived as recently as 180,000 years ago. In 1987, Cann et al. suggested that mitochondrial Eve may have lived between 140-280 thousand years ago.

Together Harvard University and XVIVO developed this 3D animation journey for Harvard’s undergraduate Molecular and Cellular Biology students about the microscopic world of mitochondria. The animation highlights the creation of Adenosine Triphosphate (ATP) — mobile molecules which store chemical energy derived from the breakdown of carbon-based food. ATP molecules act as a kind of currency, imparting chemical energy to power all the functional components of cellular activity. This piece is the second in a series of award winning animations XVIVO is creating for Harvard’s educational website BioVisions at Harvard. The first program, Inner Life of the Cell, received international acclaim and can be seen both on our website and the BioVisions site.

Implications of dating and placement of Eve

Initially there was a lot of resistance against the Mitochondrial Eve hypothesis. This resistance was due, in part, to the popularity of the Multiregional Evolution hypothesis amongst some leading paleoanthropologists such as Milford Wolpoff. This prevailing theory held that the evolution of humanity from the beginning of the Pleistocene 2.5 million years BP to the present day has been within a single, continuous human species, evolving worldwide to modern Homo sapiens. More resistance came from those who argued that there was too little time between Homo erectus and modern Homo sapiens to allow for another new species, and others who argued that for regional evolution from archaic hominin forms into modern ones. Consequently, the finding of a recent maternal ancestor for all humans in Africa was very controversial.

Cann, Stoneking & Wilson (1987)’s placement of a relatively small population of humans in sub-saharan Africa, lent appreciable support for the recent Out of Africa hypothesis. The current concept places between 1,500 and 16,000 effectively interbreeding individuals (census 4,500 to 48,000 individuals) within Tanzania and proximal regions. Later, Tishkoff et al. (2009) using data from many loci (not just mitochondrial DNA) extrapolated that the Angola-Namibia border region near the Atlantic Ocean is likely to be near the geographical point of origin of modern human genetic diversity. In its relatively southern origin proposals, this autosomal study was considered by the authors to be broadly consistent with a previous mitochondrial DNA studies, including one by some of the same authors which associated the origins of mitochondrial haplogroups L0 and L1 with “click languages” in southern and eastern Africa.

To some extent the studies have already revealed that the presence of archaic homo sapiens in Northwest Africa (Jebel Irhoud) were not likely part of the contiguous modern human population. In addition, the older remains at Skhul and Qafzeh are also unlikely part of the constrict human population, evidence currently indicates humans expanded in the region no earlier than 90,000 BP.Tishkoff argues that humans might have migrated to the Levant before 90 Ka, but this colony did not persist in SW Asia. Better defined is the genetic separation among Neanderthals, Flores hobbit, Java man, and Peking man. In 1999 Krings et al., eliminated problems in molecular clocking postulated by Nei, 1992 when it was found the mtDNA sequence for the same region was substantially different from the MRCA relative to any human sequence. Currently there are 6 fully sequenced Neanderthal mitogenomes, each falling within a genetic cluster less diverse than that for humans, and mitogenome analysis in humans has statistically markedly reduced the TMRCA range so that it no longer overlaps with Neandertal/human split times. Of all the non-African hominids European archaics most closely resembled humans, indicating a wider genetic divide with other hominids.

Since the multiregional evolution hypothesis (MREH) revolved around a belief that regional modern human populations evolved in situ in various regions (Europe: Neandertals to Europeans, Asia: Homo erectus to East Asians, Australia: Sumatran erectines to indigenous Australians), these results demonstrated that a pure MREH hypothesis could not explain one important genetic marker.

Common fallacies

Not the only woman

One of the misconceptions of Mitochondrial Eve is that since all women alive today descended in a direct unbroken female line from her that she was the only woman alive at the time. Nuclear DNA studies indicate that the size of the ancient human population never dropped below tens of thousands. Other women alive at Eve’s time have descendants alive today, but sometime in the past, each of their lines of descent included at least one male, thereby breaking the mitochondrial DNA lines of descent. By contrast, Eve’s lines of descent to each person alive today includes precisely one purely matrilineal line.

Not a contemporary of “Adam”

Sometimes Mitochondrial Eve is assumed to have lived at the same time as Y-chromosomal Adam, perhaps even meeting and mating with him. Like mitochondrial “Eve”, Y-chromosomal “Adam” probably lived in Africa; however, this “Eve” lived much earlier than this “Adam” – perhaps some 50,000 to 80,000 years earlier.

Not the most recent ancestor shared by all humans
Main article: Most recent common ancestor

Mitochondrial Eve is the most recent common matrilineal ancestor, not the most recent common ancestor (MRCA). Since the mtDNA is inherited maternally and recombination is either rare or absent, it is relatively easy to track the ancestry of the lineages back to a MRCA; however this MRCA is valid only when discussing mitochondrial DNA.

An approximate sequence from newest to oldest can list various important points in the ancestry of modern human populations:

The Human MRCA. All humans alive today share a surprisingly recent common ancestor, perhaps even within the last 5,000 years, even for people born on different continents.

The Identical ancestors point. Just a few thousand years before the most recent single ancestor shared by all living humans was the time at which all humans who were then alive either left no descendants alive today or were common ancestors to all humans alive today. In other words, “each present-day human has exactly the same set of genealogical ancestors” alive at the “Identical ancestors point” in time. This is far more recent than Mitochondrial Eve.

“Y-Chromosomal Adam”, the most recent male-line common ancestor of all living men, was much more recent than Mitochondrial Eve, but is also likely to have been long before the Identical ancestors point.

Mitochondrial Eve, the most recent female-line common ancestor of all living people.

Nomenclature

Mitochondrial Eve is named after mitochondria and the Biblical Eve. The reference to Eve may lead to the misconception that she was the only living female of her time, even though she co-existed with other females. However, all of her other female contemporaries failed to produce a direct unbroken female line to the present day.

We now know that the two key assumptions behind the data used to establish the existence of “Mitochondrial Eve” are not just flawed, but wrong. The assumption that mitochondrial DNA is passed down only by the mother is completely incorrect (it also can be passed on by the father). And, the mutation rates used so calibrate the so-called “molecular clock” are now known to have been in error. (To use the words of Rodriguez-Trelles and his coworkers, the method contains a “fundamental flaw.”)

Philip Awadalla and his coworkers noted in Science: “Many inferences about the pattern and tempo of human evolution and mtDNA evolution have been based on the assumption of clonal inheritance. There inferences will now have to be reconsidered” (1999, 286:2525). However, rather than merely “reconsidering” their theory and attempting to revamp it accordingly, evolutionists need to admit, honestly and forthrightly, that “Mitochondrial Eve,” as it turns out, has existed only in their minds, not in the facts of the real world.

A funeral and interment are in order for Mitochondrial Eve.

The molecular clock is a technique in genetics used to estimate when populations diverged. When comparing the mtDNA sequences of two populations, researchers can target mutations that are present in one population and absent in the other. The more mutations that differentiate the sequences, the longer the populations have been separated. The assumption is that mutations are random events that occur at a steady rate; for example, in humans it is sometimes estimated that a mutation takes place every 10,000 years. Thus, if mtDNA sequences of two populations differ by 5 nucleotides, it can be inferred that the two populations split from a common ancestral population 50,000 years ago. The clock can be calibrated by using a references pair of groups of living species whose date of speciation was already known from the fossil record.

The molecular clock is a statistical analysis based on many assumptions and hence its accuracy is sometimes questioned. However, studies do show some consistency with fossil records. For example, Mitochondrial Eve is calculated to have lived about 150,000 years ago. This is consistent with the emergence of anatomically modern humans based on fossil evidence. Furthermore, the dates calculated based on the Y-chromosome are in general agreement with those for mitochondrial DNA. This is a useful comparison because Y and mtDNA are inherited independently and they have different mutation rates.

Science works by analyzing the data and forming hypotheses based on those data. Science is not supposed to massage the data until they fit a certain preconceived hypothesis. All of the conclusions that have been drawn from research on Mitochondrial Eve via the molecular clock must now be discarded as unreliable.

In short, Mitochondria are components found in the cell outside the nucleus that provide energy to the cell. They are believed to have originally been free bacteria that became incorporated into the cells of organisms billions of years ago. This is because the mitochondria have their own strand of DNA in a loop similar to those of bacteria. As the mitochondria are located outside the nucleus they do not participate in the shuffling of DNA that occurs during reproduction. When the sperm fuses with an egg cell during fertilization, the sperm’s mitochondria are destroyed, leaving only the egg cell’s mitochondria in the new fertilized zygote. This process only passes down the mother’s mitochondrial DNA (mtDNA) to the next generation. All people have their mother’s mtDNA but not their father’s. Thus, a family tree of humanity based on the maternal line can be drawn, whereby the tree eventually coalesces on one female, and that person is Mitochondrial Eve.

The DNA in mitochondria is relatively short with only 16,569 base pairs. Thus it is much easier to study than the rest of the genome. When a woman’s mitochondria are copied and packed into an egg cell, the DNA is almost always the same. However, every once in a while a mutation takes place which alters the sequence of the DNA strand.

If Mitochondrial Eve had two daughters, one of whom happened to have a mutation in her Mitochondrial DNA, then all the women alive today descended from that daughter would share that mutation. All the women descended from the other daughter would not. Thus mutations in mtDNA are useful in determining lineages and migratory patterns.

Well women, what is your opinion on this; “Do we have the funeral or not?” 🙂

Comments

I was please to have my mtDNA tested. The results were my maternal line is 100% Mende Tribe from Sierra Leone. I means a lot to me and my family to know our ancestry and kinship from the past. Thanks for post this article. It has help to enlighten me even more about our maternal past.